The use of a gas-cluster ion beam (GCIB) for etching, cleaning, and smoothing surfaces is known in the art (see, for example, U.S. Pat. No. 5,814,194, Deguchi et al., incorporated herein by reference). GCIBs have also been employed for assisting the deposition of films from vaporized carbonaceous materials (see, for example, U.S. Pat. No. 6,416,820, Yamada et al., incorporated herein by reference). For purposes of this discussion, gas-clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters may consist of aggregates of from a few to several thousand molecules or more that are loosely bound to form a cluster. The clusters can be ionized by electron bombardment, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q·e (where e is the magnitude of the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). The larger sized cluster-ions are often the most useful because of their ability to carry substantial energy per cluster-ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently, the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes gas-cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage, which is characteristic of conventional ion beam processing.
Means for creation of and acceleration of such GCIBs are described in the reference (U.S. Pat. No. 5,814,194) previously cited. Presently available cluster-ion sources produce cluster-ions having a wide distribution of sizes, N, up to N of several thousand (where N=the number of molecules in each cluster). In the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, a molecular ion, or a monomer ion.
Many useful surface-processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not limited to, smoothing, etching, film growth, and infusion of materials into surfaces. In many cases, it is found that in order to achieve industrially practical throughputs in such processes, GCIB currents on the order of hundreds or perhaps thousands of microamps are required. Ionizers for ionizing gas-cluster jets to form GCIBs have historically been of the electron impact type, utilizing thermoelectrons to ionize gas-clusters by impact with the clusters. Such impact often ejects one or more electrons from a gas-cluster, leaving it positively charged. In U.S. Pat. No. 6,629,508 (incorporated herein by reference), Dykstra has described several forms of prior art ionizers for GCIB formation.
Several emerging applications for GCIB processing of workpieces on an industrial scale are in the semiconductor field. Due to yield considerations, such applications typically require that processing steps contribute only very low levels of contamination. Although GCIB processing of workpieces is done using a wide variety of gas-cluster source gases, many of which are inert gases, in many semiconductor processing applications it is desirable to use reactive source gases in the formation of GCIBs, sometimes in combination or mixture with inert or noble gases. Often halogen-containing gases, oxygen, and other reactive gases or mixtures thereof are incorporated into GCIBs, sometimes in combination or mixture with inert or noble gases. These reactive gases pose a problem for gas-cluster ionizer design for semiconductor processing because of their corrosive nature. For example, NF3 and O2 are often combined for forming GCIBs for use in etching or smoothing processes. When gas-clusters comprising NF3 and O2 are bombarded by electrons during ionization to form gas-cluster ions, there is a certain amount of evaporation of the gas-clusters that results, evolving corrosive gas components inside the ionizer. Since thermionic filaments used to generate thermoelectrons for ionization operate at high temperatures, they are susceptible to attack by the reactive and corrosive gases that are evolved. Particularly when the partial pressures of the reactive gases reach high levels, such attack can be exacerbated and can extend to other lower temperature materials-of-construction of the ionizer as well as to the high temperature filaments. This type of reactive interaction of corrosive gas constituents with the ionizer filament(s) and other materials in the ionizer has two important drawbacks. It shortens the operational lifetime of the filaments in the ionizers, and it results in the generation of small particles and molecules or ions of potential contaminants that can interfere with the yield of semiconductor processes. Specifically, since the thermionic filaments are normally metallic (often tungsten) and since other materials-of-construction of the conventional ionizers are also often metal (aluminum, iron, molybdenum, etc.), the use of reactive gas components in the gas-clusters results in generation of ions, molecules, and small particles of metals and compounds containing metal atoms. In a GCIB processing tool for application in the field of semiconductor processing, or in any field of application requiring low levels of contamination, such generation of contaminants is detrimental to the GCIB processing because such contaminants inevitably transport from the ionizer to the surfaces of the workpieces being processed.
In order to produce high ionization efficiency in a GCIB ionizer, it has been desirable to have available a high electron flux for impact ionization of gas-clusters. In order to produce high emission currents in thermionic emission electron sources, high electric fields are commonly employed to overcome space-charge effects that otherwise tend to limit thermionic emission. An undesirable side effect of this expedient is that the energies of the impact ionization ionizing electrons are higher than desirable, often several hundred electron volts. High energy electrons may contribute to the production of higher ionization states of multiply ionized gas-cluster ions. In some GCIB processing applications, large quantities of highly ionized (ionization state above 3 or 4) gas-cluster ions are considered detrimental to the process.